To study prevalence of brown marmorated stink bug (Halyomorpha halys) in host trees, researchers led by Nicole Quinn at Virginia Tech used an eye bolt and pulley system so that the pheromone-baited traps could be raised or lowered along the height of the tree. Their findings suggest that monitoring efforts for the stink bug and its egg parasitoid wasp Trissolcus japonicus should seek higher placement in tree canopies and that ground-based traps may miss or underestimate presence of both the pest and its parasitoid. (Photo credit: Kristie Graham, USDA ARS, Bugwood.org)

By Andrew Porterfield

The brown marmorated stink bug (Halyomorpha halys) was accidentally introduced to North America around 1998, and has since settled in 44 states and four Canadian provinces, as well as Europe and South America, becoming a household nuisance and major agricultural pest.

Andrew Porterfield

H. halys feeds on as many as 60 species of crops and wild plants as a sucking insect, using its proboscis to pierce and feed on a host plant. The insect has been behind tens of millions of dollars in losses to apples, peaches, and other commodities. Because it moves from crop to crop from wild host plants and generally feeds on the borders of crop areas, insecticidal treatments on the stink bug do not have much effect on its behavior. In addition, many of the insecticides used on H. halys are toxic to the insect’s natural enemies, so treatments can significantly impair integrated pest management practices.

A number of parasites and predators attack H. halys, but none has been fully effective at keeping the invasive insect population in check. One promising predator, the Asian wasp Trissolcus japonicus (also known as the samurai wasp), the eggs of which feed on H. halys eggs, was found recently in samples from the Mid-Atlantic United States.

At the same time, some experiments have begun showing differences in crop damage from brown marmorated stink bugs, varying according to the height of fruit trees. These studies have shown that H. halys may be most destructive in the upper canopy of trees. The finding could mean that the stink bug’s egg masses (and parasitoids that prey on those eggs) may also be more prevalent in the tops of trees. But current monitoring of stink bug populations has typically focused on ground-based traps, suggesting that, when looking for ways to control H. halys, scientists may be looking in the wrong place.

To determine the optimal locations for monitoring H. halys, Nicole Quinn, a graduate researcher at Virginia Tech, and colleagues at the Florida Department of Agriculture and Consumer Services, the University of Georgia, and the USDA Agricultural Research Service designed a unique way to measure stinkbug and predator populations at various heights in host trees. Their research was published in December in the journal Environmental Entomology.

The scientists used host trees to trap H. halys at Virginia Tech’s Agricultural Research and Extension Center in Virginia. The tree of heaven (Ailanthus altissima) and hackberry (Celtis occidentalis) were used because they are the most common deciduous trees in the region and are well known for hosting stinkbugs. Pheromone traps were set up on each tree, using an eye bolt and pulley system so that the traps could be raised or lowered along the height of the trunk. The researchers also felled trees of heaven growing at the woods’ edge next to commercial fruit orchards, to measure egg masses in the trees. They measured weekly trap results from spring until fall, for two years (2016 and 2017).

To study prevalence of brown marmorated stink bug in trees, researchers led by Nicole Quinn at Virginia Tech placed pheromone-baited pyramid traps in trees (tree of heaven, Ailanthus altissima, shown here) near the ground, mid-canopy, and near the top of the canopy. Combined with felling trees to collect stink bug egg masses, the researchers found greater stink bug presence in the middle and upper levels, and stink bug egg masses parasitized by the wasp Trissolcus japonicus were found only in middle and upper levels, as well. (Images originally published in Quinn et al 2018, Environmental Entomology)

Quinn and her colleagues found significant variations in population, depending on the vertical location of the stinkbug pheromone trap. The largest numbers of adults were trapped in upper canopy locations, and the smallest number were trapped in lower canopy traps. Significantly more nymphs were trapped in the upper canopy than lower and mid-canopy traps (while mid- and lower-canopy nymph populations did not differ significantly).

For egg masses, 51 H. halys egg masses were collected during the course of the study. Most masses (59 percent) were in the middle third of tree canopies, and all egg masses collected either produced nymphs or parasitoids or showed signs of predators. The most prevalent parasitoid that emerged was T. japonicus. Detection of T. japonicus correlated positively with tree height and dry leaf biomass, as well as with branch height.

The brown marmorated stink bug (Halyomorpha halys) lays its eggs in a mass on the underside of leaves. A recent study led by Nicole Quinn at Virginia Tech sampled for egg masses in host trees and analyzed their vertical location, finding most (59 percent) in the middle height range. (Photo credit: Jennifer Carr, University of Florida, Bugwood.org)

The study is believed to be the first to measure vertical distribution of both H. halys and its T. japonicus predator in trees. “Previously, the only information about the distribution of H. halys adults and nymphs in the canopy was from anecdotal observations suggesting a higher density in the upper portion of wild hosts. Furthermore, that was no information on the distribution of H. halys egg masses, the resource for T. japonicus, in the tree,” the researchers write.

Since T. japonicus was detected only in egg masses from the mid- and upper-canopy of sample trees, ground-based sampling could easily be underestimating the presence of T. japonicus and its parasitization of H. halys eggs. The study shows how T. japonicus could be more strategically used to reduce egg mass populations. In addition, the researchers note that since the study was concluded, populations of T. japonicus have continued to grow, “boding well for its potential to impact H. halys populations,” the researchers conclude.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.